1. Field of the Invention
The present invention relates to an autofocusing apparatus which detects focus by a charge storage type photo-electric converter such as a CCD and drives a photographing lens into an in-focus position.
2. Related Background Art
An auto-focusing apparatus which directs a portion of a light beam from an object and transmitted through a lens to a charge storage type photo-electric converter having a plurality of photo-sensing elements such as a CCD, detects focus in accordance with an output signal of the photo-electric converter which represents a light intensity distribution of an object image, and drives a photographing lens into an in-focus position in accordance with the result of the focus detection, has been known.
In such an apparatus, an object image signal produced by the photo-electric converter is amplified to an appropriate signal level by an amplifier, it is A/D converted and it is supplied to a microcomputer which processes it for detecting the focus. It is desired that the object image signal used for the focus detection has a certain signal level in order to secure a focus detection precision. However, when a light intensity of the object is not constant but it significantly changes or a photographer selects another object, the light intensity of the object may significantly change by a factor of several thousands to ten thousands. Accordingly, the charge storage time of the photo-electric converter, a stored charge-voltage conversion gain of the photo-electric converter itself and a gain of a succeeding stage amplifier are adjusted in accordance with the change of the light intensity to transfer a predetermined level of object image signal to the microcomputer.
FIG. 5 shows an output voltage of a photosensing element (1.about.n) amplified to an appropriate signal level, that is, an input voltage to the A/D converter. A target input voltage Da is so set that an input voltage may not exceed the rated input voltage of 10V for a normal variation of the object image signal and that a reduction of an S/N ratio due to a noise may not be caused. For example, when the A/D converter has a rated input voltage of 10V and an 8-bit output (0-255), the target input voltage Da is set to 7V.
However, when the change of the light intensity of the object is so abrupt that the adjustment of the signal level cannot follow, a portion of the object image signal may exceed the rated input voltage of 10V of the A/D converter as shown in FIG. 6. In this case, an output for the object image signal which overflows the input range of the A/D converter is 255. If the focus detection is done in accordance with such an object image signal, the focus detection precision decreases so that the photographing lens may not be driven to the in-focus position. Thus, in the prior art apparatus, if a portion of the object image signal overflows, the focus detection based on the object image signal and the focus adjustment of the photographing lens based on the focus detection are inhibited, and the charge storage time and the amplifier again are adjusted and the charge storage of the photo-electric converter is started again to bring the object image signal level at the next storage to the target level. As a result, the object image signal having a proper level as shown in FIG. 5 is produced and a defocus amount is calculated by a predetermined algorithm in accordance with the object image signal, and the photographing lens is driven in accordance with the defocusing amount.
There is no much difference between the defocusing amounts calculated by the focus detection in accordance with the object image signal a portion of which overflows the input range of the A/D converter as shown in FIG. 6 and by the focus detection in accordance with the object image signal having the target level derived from the same object by adjusting the charge storage time and the amplifier gain as shown in FIG. 5.
However, in the prior art auto-focusing apparatus, when a portion of the object image signal overflows, the focus detection based on such data and the focus adjustment of the photographing lens based on the focus detection are inhibited and the charge storage time and the amplifier gain are adjusted to resume the charge storage. As a result, the photographing lens is not driven until the next object image signal is produced by the photo-electric converter, and the delay time when the lens is driven increases and the operability is deteriorated.